1. Field of the Invention
This invention relates to an optical bus for transmitting an optical signal, a method of producing the optical bus, and a signal processor for carrying out signal processing including data transmission and reception using the optical bus.
2. Related Art
The development of very large scale integrated circuits (VLSI) have greatly increased the number of circuit functions of a daughter board used in a data processing system. Since the number of signal connections for each daughter board grows along with an increase in the number of circuit functions, parallel architecture which requires a large number of connectors and connection lines is now employed in a data bus board (mother board) for connecting the daughter boards by a bus structure. The operation speed of a parallel bus is increased by employing the parallel architecture by reducing the width of each connection line and increasing the number of layers for connection lines. However, the processing speed of a system may be limited by the operation speed of the parallel bus due to a signal delay caused by capacity between connection lines or connection line resistance. Electromagnetic interference (EMI) caused by increasing the density of parallel bus connection lines is a great limitation to the improvement of the processing speed of the system.
Use of an intra-system optical connection technology called "optical interconnection" is now under study to solve the above problems and improve the operation speed of a parallel bus. As outlined by Teiji Uchida, the 9th Circuit Packaging Technology Lecture Meeting, 150C011, pp.201 to 202, H. Tomimuro et al., "Packaging Technology for Optical Interconnects", IEEE Tokyo No. 33, pp.81-86, 1994, and Osamu Wada, Electronics Vol. 4, pp.52-55, 1993, there are proposed various optical interconnection technologies according to system configuration.
Out of conventionally proposed optical interconnection technologies, Japanese Published Patent Application No. Hei 6-22351 (discloses the application of an optical data transmission system using high-speed and high-sensitivity light emitting/receiving devices in a data bus. This publication proposes a serial optical data bus for loop transmission between circuit boards in which light emitting/receiving devices are arranged on both front and rear sides of each circuit board and the light emitting/receiving devices on adjacent circuit boards incorporated in a system frame are spatially and optically interconnected. In this system, signal light transmitted from a circuit board is converted into an electric signal by an adjacent circuit board and the electric signal is further converted into an optical signal by the circuit board to be transmitted to the next circuit board. Thus, circuit boards are arranged in series, and opto-electric conversion and electro-optic conversion are carried out on each of the circuit boards so that the signal is transmitted to all the circuit boards incorporated in the system frame. Therefore, the signal transmission speed depends on and is limited by the opto-electric conversion speed and electro-optic conversion speed of the light emitting/receiving devices disposed on each of the circuit boards. Since the optical interconnection of light emitting/receiving devices on the circuit boards with a free space between them is used for data transmission between circuit boards, all the circuit boards must be optically interconnected by carrying out optical alignment of the light emitting/receiving devices on both front and rear sides of adjacent circuit boards. Further, since the circuit boards are interconnected with a free space between them, a data transmission failure may occur due to a crosstalk between adjacent optical data transmission lines. A data transmission failure may also occur due to diffusion of signal light caused by an environment of the system frame, such as dust. Since the circuit boards are arranged in series, when any one of the circuit boards is removed, the interconnection is broken. To compensate for this, an extra circuit board is necessary. That is, a circuit board cannot be removed or added freely and the number of circuit boards is fixed.
A data transmission technology between circuit boards using 2-D array devices is disclosed by Japanese Published Patent Application No. Hei 6-93051. The technology disclosed herein is to optically interconnect circuit boards through an optical path comprising a plate having two parallel surfaces and arranged to face a light source, a diffraction grating and reflection elements, both arranged on the surface of the plate. In this system, light emitted from one point can be transmitted to only one fixed point and all the circuit boards cannot be interconnected unlike an electric bus. Further, the system requires a complex optical system and it is difficult to align optical elements. Therefore, a data transmission failure may occur due to a crosstalk between adjacent optical data transmission paths caused by displacement of the optical elements. Since information on the interconnection of circuit boards is determined by the diffraction grating and reflection elements arranged on the surface of the plate, a circuit board cannot be removed or added with the result that extendability is low.
Another data transmission technology between circuit boards using 2-D array devices is disclosed by Japanese Published Unexamined Patent Application No. Hei 4-134415. This publication discloses a data transmission system comprising a substrate made of a transparent material having a higher refractive index than air, a lens array consisting of a plurality of lenses having a negative curvature and an optical system for causing light emitted from a light source to be incident upon the side surface of the lens array. This publication also discloses a system for forming an area having a low refractive index or a hologram in the substrate in place of the plurality of lenses having a negative curvature. In these systems, light input from the side surface of the substrate is distributed over the top surface of the substrate and output from the plurality of lenses having a negative curvature, the area having a low refractive index or a portion forming the hologram in place of these lenses. Therefore, it is conceivable that the intensity of an emitted signal may vary according to the relationship between the light input position and the light output position of the plurality of lenses, the area having a low refractive index or the portion of the substrate forming the hologram in place of the lenses. The probability of light input from the side surface of the substrate going through a side surface facing the light input surface is considered to be high and the efficiency of light used for signal propagation is low. Further, since optical input elements of the circuit boards must be arranged at the position of the plurality of lenses having a negative curvature, the area having a low refractive index, or the hologram in place of these lenses on the substrate, the freedom of the arrangement of the circuit boards is small and the extendability of the system is low.
It is conceivable to employ an optical bus for diffusing incident light and propagating it as means for solving these problems. However, when the optical bus is formed by laminating together a plurality of light transmission layers for diffusing input light and propagating it so that light is output from an end surface of each light transmission layer as this optical bus and light output from this end surface is received by light receiving elements provided near the end surface, if the thickness of each light transmission layer is small, it is difficult to align the surface of each light transmission layer from which the signal light is output with the light receiving surface of each light receiving element and a data transmission failure occurs. On the other hand, if the thickness of each light transmission layer is made large, the diffusion of signal light output from the light transmission layer is large in the thickness direction of the light transmission layer, thereby reducing the use efficiency of light.